IN-VITRO FLOW CHARACTERISTICS OF THE AHMED AND SELF-CONSTRUCTED ANTERIOR-CHAMBER SHUNTS

Objective-To compare in vitro opening pressures (OP) and closing press ures (CP) of the Ahmed VS-1 and VS-2 glaucoma valves with those of sev eral self-constructed valve 'prototypes,' and to assess their ability to maintain perfusion pressures between 6 and 21 mm of Hg. Sample Popu lation-Ahmed VS-1 (n = 6), 2 groups of Ahmed VS-2 (group 1: n = 12; gr oup 2: n = 14), and self-constructed Valves with linear incisions in t he long axis of the tube wail (n = 6) or X-shaped incisions in the tub e wails (n = 2). Procedure-Valves were perfused with deionized water, lactated Ringer's solution (LRS), Dulbecco's modified Eagle's medium ( DMEM), DMEM plus 50% equine serum (ES), and 100% ES. Flow rates of 2.8 5, 4.2, 6.0, 9.0, and 12.0 mu l/min were used for each perfusate. Valv es were tested 3 times for reproducibility, and OP/CP were compared fo r each system. Results-OP/CP of the VS-1, VS-2 (group 1), VS-2 (group 2), and linear 1.0-cm incisional Valves with thick tubing consistently increased with increasing perfusion rate, linear 0.5-cm (thick tubing ) and 1.0-cm (thin tubing) incisional valves had increasing OP/CP with increasing perfusion rate in all but a few instances. Mean OP/CP decr eased with increasing perfusate osmolarity for all perfusates except L RS, using the VS-1 and V-2 (group 2) valves. Mean OP/CP were consisten tly lower for VS-1 than VS-2 (group 1) valves at any given flow rate a nd for any given perfusate. Mean OP/ CP were consistently lower for VS -2 (group 1) than VS-2 (group 2) valves at any given flow rate and for any given perfusate. The linear 0.5-cm incisional valves with thick a nd thin tubing induced the highest mean OP/CP, maximizing at > 30 mm o f Hg. Conclusions-Only the VS-2 (group 2) valves consistently had mean OP/CP between 6 and 21 mm of Hg for all perfusates and at all flow ra tes. Clinical Relevance-Anterior chamber shunts, although imperfect, a ppear to offer a physiologically sound alternative for glaucoma manage ment.